Field of the Invention
The present invention relates to a wire grid polarizer capable of improving light efficiency by applying a nano-wire grid pattern optimized to a LCD panel, a method of manufacturing thereof, and a liquid crystal display panel and a liquid crystal display device provided with the wire grid polarizer.
Background of the Related Art
Generally, a liquid crystal display (LCD) device is an electronic element which converts various kinds of electrical information generated by a variety of apparatuses into visual information using changes in transmittance of liquid crystal depending on an applied voltage.
Since the LCD device is advantageous in miniaturization, lightweightness and low power consumption, it is spotlighted as an alternative means that can overcome disadvantages of cathode ray tubes (CRTs) widely used in the past and is currently mounted on most of information processing devices that need a display device.
FIG. 1 is a view showing the structure of a conventional LCD device, in which the LCD device 10 includes a thin film transistor (TFT) substrate 11 formed with a gate line, a data line, TFTs and pixel electrodes, a color filter substrate 12 disposed to be opposite to the TFT substrate 11 and formed with a color filter and a common electrode, and a liquid crystal layer 13 filled between the TFT substrate 11 and the color filter substrate 12.
The TFT substrate 11 is a transparent glass substrate formed with thin film transistors on a matrix, in which the data line is connected to the source terminal, and the gate line is connected to the gate terminal. In addition, the pixel electrodes 11a made of transparent indium tin oxide (ITO), which is a conductive material, are formed at the drain terminal. The color filter substrate 12 is disposed over the TFT substrate 11 to face the TFT substrate 11. The color filter substrate 12 is a substrate on which R, G, and B pixels, which are color pixels emitting a certain color when light passes through, are formed through a thin film process, and the common electrode 12a made of ITO is formed on the front side thereof. In addition, polarizing plates 16 and 17 for polarizing unpolarized light supplied by a light source into linearly polarized light are provided under and on the TFT substrate 11 and the color filter substrate 12, respectively. The polarizing plates 16 and 17 maintain a penetrating direction of light to be constant depending on the alignment direction of the liquid crystal layer 13, and a reflective polarizing plate (DBEF or WGP) 20 that can enhance reusability of light by passing light of a specific polarizing direction and reflecting light of other polarizing directions is provided in addition to the polarizing plates 16 and 17.
Meanwhile, since the liquid crystal provided in the liquid crystal layer 13 is a light receiving element, the LCD device 100 needs a part which provides light separately. A backlight unit 18 is separately formed on the rear side of the TFT substrate 11 in order to provide the light. A lamp for providing light, a light guide plate for evenly distributing the light on all over the substrate, and other films are formed in the backlight unit 18.
The LCD device 10 configured as described above does not pass all the light provided by the backlight unit 18, and thus brightness is very important. A variety of films are developed and used in order to improve brightness of the LCD device 10, and a typical example thereof is a reflective polarizing film (polarizing plate).
Recently, such a reflective polarizing film takes an important role in the display industry, which is one of national core industries. There are various kinds of reflective polarizing films, and a dual brightness enhancement film (DBEF) or a diffusive reflective polarization film (DRPF) are typical examples thereof. The DBEF is a film where isotropic films and anisotropic films are repeatedly formed to have a stacked structure of hundreds of layers (about six hundreds or more layers). The light passing through the film is increased in total when the light passes through and is reflected in the stacked structure of hundreds or more layers, and thus brightness of the LCD device is improved. On the other hand, the DRPF is formed with another material having a refractive index different from that of the DRPF, and thus light passing through the film is increased since the light is reflected and refracted by the material.
Since the DBEF has the highest brightness improvement ratio among the reflective polarizing films, the DBEF is advantageous in enhancing characteristics of light efficiency when it is applied to a LCD device. However, since such a DBEF cannot be regarded as a complete polarizer element and a stacked structure of hundreds of thin film layers should be formed in manufacturing the DBEF, the manufacturing process is complicated, and the manufacturing cost is very high, and thus the DBEF is difficult to be used in a low price LCD device.
Accordingly, as is shown in Korean Laid-open Patent No. 10-2007-0101814, use of a wire grid polarizer (WGP) is proposed recently as a substitute for the DBEF, in which the WGP is a polarizer element which passes light of a specific polarizing direction and reuses light of the other polarizing directions by reflecting the light. Since such a wire grid polarizer has a high polarization splitting performance compared with those of the other polarizer, it can be advantageously used as a reflective polarizer.
FIGS. 2 and 3 are a perspective view and a side view showing a conventional wire grid polarizer (WGP). As shown in the figures, the conventional wire grid polarizer 20 is an element for generating polarized light using a conductive wire grid, which has a structure formed with a wire grid pattern 22 where a nano size wire of a conductive material is arranged in parallel at regular intervals. Since such a wire grid polarizer 20 does not generate diffraction if the interval of the wire grid is smaller than the wavelength of incident light, wire grid polarizer 20 passes components having a vibration direction perpendicular to the conductive wire grid among the incident light, i.e., transverse magnetic (TM) polarized light, and reflects components having a vibration direction parallel to the wire grid, i.e., transverse electric (TE) light.
However, the conventional wire grid polarizer 20 described above is provided with a wire grid pattern of the same shape on all the area of the substrate 21 and has a structure where a single wire grid pattern of the same shape is uniformly applied to all wavelength bands of visible light, and thus it may obtain comparatively superior light efficiency from the light of a specific wavelength. However, light efficiency of the light of the other wavelengths is not favorable. That is, as shown in FIG. 4, in the case of light polarized through the wire grid polarizers 20a and 20b disposed under and on the liquid crystal layer 13 in the LCD device 10, only the light of a specific wavelength component (e.g., arrow G in the figure) has a high transmittance, and light of the other wavelength components (e.g., arrows R and B in the figure) does not have a good transmittance, and thus light efficiency thereof is lowered. As described, the conventional wire grid polarizer 20 is not optimally designed for respective colors of red, green and blue of the color filter substrate 12, and thus the light efficiency is favorable only in a specific wavelength band, and the overall light efficiency is lowered. Furthermore, since there is a technical limit in manufacturing a wide area nano-wire pattern, development of a reflective polarizer element of a new concept for improving light efficiency is desperately required.